[java] java 中Unsafe类学习

java不能直接访问操作系统底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作，主要提供了以下功能：

1、通过Unsafe类可以分配内存，可以释放内存；

类中提供的3个本地方法allocateMemory、reallocateMemory、freeMemory分别用于分配内存，扩充内存和释放内存，与C语言中的3个方法对应。

2、可以定位对象某字段的内存位置，也可以修改对象的字段值，即使它是私有的；

1. public native long allocateMemory(long l);
2. public native long reallocateMemory(long l, long l1);
3. public native void freeMemory(long l);

字段的定位：JAVA中对象的字段的定位可能通过staticFieldOffset方法实现，该方法返回给定field的内存地址偏移量，这个值对于给定的filed是唯一的且是固定不变的。getIntVolatile方法获取对象中offset偏移地址对应的整型field的值,支持volatile load语义。getLong方法获取对象中offset偏移地址对应的long型field的值数组元素定位：Unsafe类中有很多以BASE_OFFSET结尾的常量，比如ARRAY_INT_BASE_OFFSET，ARRAY_BYTE_BASE_OFFSET等，这些常量值是通过arrayBaseOffset方法得到的。arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址。Unsafe类中还有很多以INDEX_SCALE结尾的常量，比如 ARRAY_INT_INDEX_SCALE ， ARRAY_BYTE_INDEX_SCALE等，这些常量值是通过arrayIndexScale方法得到的。arrayIndexScale方法也是一个本地方法，可以获取数组的转换因子，也就是数组中元素的增量地址。将arrayBaseOffset与arrayIndexScale配合使用，可以定位数组中每个元素在内存中的位置。

1. public final class Unsafe {
2. public static final int ARRAY_INT_BASE_OFFSET;
3. public static final int ARRAY_INT_INDEX_SCALE;
4. public native long staticFieldOffset(Field field);
5. public native int getIntVolatile(Object obj, long l);
6. public native long getLong(Object obj, long l);
7. public native int arrayBaseOffset(Class class1);
8. public native int arrayIndexScale(Class class1);
9. static
10. {
11. ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset([I);
12. ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale([I);
13. }
14. }

3、挂起与恢复

将一个线程进行挂起是通过park方法实现的，调用 park后，线程将一直阻塞直到超时或者中断等条件出现。unpark可以终止一个挂起的线程，使其恢复正常。整个并发框架中对线程的挂起操作被封装在 LockSupport类中，LockSupport类中有各种版本pack方法，但最终都调用了Unsafe.park()方法。

1. public class LockSupport {
2. public static void unpark(Thread thread) {
3. if (thread != null)
4. unsafe.unpark(thread);
5. }
6. public static void park(Object blocker) {
7. Thread t = Thread.currentThread();
8. setBlocker(t, blocker);
9. unsafe.park(false, 0L);
10. setBlocker(t, null);
11. }
12. public static void parkNanos(Object blocker, long nanos) {
13. if (nanos > 0) {
14. Thread t = Thread.currentThread();
15. setBlocker(t, blocker);
16. unsafe.park(false, nanos);
17. setBlocker(t, null);
18. }
19. }
20. public static void parkUntil(Object blocker, long deadline) {
21. Thread t = Thread.currentThread();
22. setBlocker(t, blocker);
23. unsafe.park(true, deadline);
24. setBlocker(t, null);
25. }
26. public static void park() {
27. unsafe.park(false, 0L);
28. }
29. public static void parkNanos(long nanos) {
30. if (nanos > 0)
31. unsafe.park(false, nanos);
32. }
33. public static void parkUntil(long deadline) {
34. unsafe.park(true, deadline);
35. }
36. }

4、CAS操作

是通过compareAndSwapXXX方法实现的

1. /**
2. * 比较obj的offset处内存位置中的值和期望的值，如果相同则更新。此更新是不可中断的。
3. *
4. * @param obj 需要更新的对象
5. * @param offset obj中整型field的偏移量
6. * @param expect 希望field中存在的值
7. * @param update 如果期望值expect与field的当前值相同，设置filed的值为这个新值
8. * @return 如果field的值被更改返回true
9. */
10. public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);

 CAS操作有3个操作数，内存值M，预期值E，新值U，如果M==E，则将内存值修改为B，否则啥都不做。 另外附上Unsafe 源代码：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//package sun.misc;import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.ProtectionDomain;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;public final class Unsafe {
    private static final Unsafe theUnsafe;
    public static final int INVALID_FIELD_OFFSET = -1;
    public static final int ARRAY_BOOLEAN_BASE_OFFSET;
    public static final int ARRAY_BYTE_BASE_OFFSET;
    public static final int ARRAY_SHORT_BASE_OFFSET;
    public static final int ARRAY_CHAR_BASE_OFFSET;
    public static final int ARRAY_INT_BASE_OFFSET;
    public static final int ARRAY_LONG_BASE_OFFSET;
    public static final int ARRAY_FLOAT_BASE_OFFSET;
    public static final int ARRAY_DOUBLE_BASE_OFFSET;
    public static final int ARRAY_OBJECT_BASE_OFFSET;
    public static final int ARRAY_BOOLEAN_INDEX_SCALE;
    public static final int ARRAY_BYTE_INDEX_SCALE;
    public static final int ARRAY_SHORT_INDEX_SCALE;
    public static final int ARRAY_CHAR_INDEX_SCALE;
    public static final int ARRAY_INT_INDEX_SCALE;
    public static final int ARRAY_LONG_INDEX_SCALE;
    public static final int ARRAY_FLOAT_INDEX_SCALE;
    public static final int ARRAY_DOUBLE_INDEX_SCALE;
    public static final int ARRAY_OBJECT_INDEX_SCALE;
    public static final int ADDRESS_SIZE;    private static native void registerNatives();    private Unsafe() {
    }    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class var0 = Reflection.getCallerClass();
        if(var0.getClassLoader() != null) {
            throw new SecurityException("Unsafe");
        } else {
            return theUnsafe;
        }
    }    public native int getInt(Object var1, long var2);    public native void putInt(Object var1, long var2, int var4);    public native Object getObject(Object var1, long var2);    public native void putObject(Object var1, long var2, Object var4);    public native boolean getBoolean(Object var1, long var2);    public native void putBoolean(Object var1, long var2, boolean var4);    public native byte getByte(Object var1, long var2);    public native void putByte(Object var1, long var2, byte var4);    public native short getShort(Object var1, long var2);    public native void putShort(Object var1, long var2, short var4);    public native char getChar(Object var1, long var2);    public native void putChar(Object var1, long var2, char var4);    public native long getLong(Object var1, long var2);    public native void putLong(Object var1, long var2, long var4);    public native float getFloat(Object var1, long var2);    public native void putFloat(Object var1, long var2, float var4);    public native double getDouble(Object var1, long var2);    public native void putDouble(Object var1, long var2, double var4);    /** @deprecated */
    @Deprecated
    public int getInt(Object var1, int var2) {
        return this.getInt(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putInt(Object var1, int var2, int var3) {
        this.putInt(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public Object getObject(Object var1, int var2) {
        return this.getObject(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putObject(Object var1, int var2, Object var3) {
        this.putObject(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public boolean getBoolean(Object var1, int var2) {
        return this.getBoolean(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putBoolean(Object var1, int var2, boolean var3) {
        this.putBoolean(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public byte getByte(Object var1, int var2) {
        return this.getByte(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putByte(Object var1, int var2, byte var3) {
        this.putByte(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public short getShort(Object var1, int var2) {
        return this.getShort(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putShort(Object var1, int var2, short var3) {
        this.putShort(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public char getChar(Object var1, int var2) {
        return this.getChar(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putChar(Object var1, int var2, char var3) {
        this.putChar(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public long getLong(Object var1, int var2) {
        return this.getLong(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putLong(Object var1, int var2, long var3) {
        this.putLong(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public float getFloat(Object var1, int var2) {
        return this.getFloat(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putFloat(Object var1, int var2, float var3) {
        this.putFloat(var1, (long)var2, var3);
    }    /** @deprecated */
    @Deprecated
    public double getDouble(Object var1, int var2) {
        return this.getDouble(var1, (long)var2);
    }    /** @deprecated */
    @Deprecated
    public void putDouble(Object var1, int var2, double var3) {
        this.putDouble(var1, (long)var2, var3);
    }    public native byte getByte(long var1);    public native void putByte(long var1, byte var3);    public native short getShort(long var1);    public native void putShort(long var1, short var3);    public native char getChar(long var1);    public native void putChar(long var1, char var3);    public native int getInt(long var1);    public native void putInt(long var1, int var3);    public native long getLong(long var1);    public native void putLong(long var1, long var3);    public native float getFloat(long var1);    public native void putFloat(long var1, float var3);    public native double getDouble(long var1);    public native void putDouble(long var1, double var3);    public native long getAddress(long var1);    public native void putAddress(long var1, long var3);    public native long allocateMemory(long var1);    public native long reallocateMemory(long var1, long var3);    public native void setMemory(Object var1, long var2, long var4, byte var6);    public void setMemory(long var1, long var3, byte var5) {
        this.setMemory((Object)null, var1, var3, var5);
    }    public native void copyMemory(Object var1, long var2, Object var4, long var5, long var7);    public void copyMemory(long var1, long var3, long var5) {
        this.copyMemory((Object)null, var1, (Object)null, var3, var5);
    }    public native void freeMemory(long var1);    /** @deprecated */
    @Deprecated
    public int fieldOffset(Field var1) {
        return Modifier.isStatic(var1.getModifiers())?(int)this.staticFieldOffset(var1):(int)this.objectFieldOffset(var1);
    }    /** @deprecated */
    @Deprecated
    public Object staticFieldBase(Class var1) {
        Field[] var2 = var1.getDeclaredFields();        for(int var3 = 0; var3 < var2.length; ++var3) {
            if(Modifier.isStatic(var2[var3].getModifiers())) {
                return this.staticFieldBase(var2[var3]);
            }
        }        return null;
    }    public native long staticFieldOffset(Field var1);    public native long objectFieldOffset(Field var1);    public native Object staticFieldBase(Field var1);    public native boolean shouldBeInitialized(Class<?> var1);    public native void ensureClassInitialized(Class var1);    public native int arrayBaseOffset(Class var1);    public native int arrayIndexScale(Class var1);    public native int addressSize();    public native int pageSize();    public native Class defineClass(String var1, byte[] var2, int var3, int var4, ClassLoader var5, ProtectionDomain var6);    /** @deprecated */
    @Deprecated
    @CallerSensitive
    public native Class defineClass(String var1, byte[] var2, int var3, int var4);    public native Class defineAnonymousClass(Class var1, byte[] var2, Object[] var3);    public native Object allocateInstance(Class var1) throws InstantiationException;    public native void monitorEnter(Object var1);    public native void monitorExit(Object var1);    public native boolean tryMonitorEnter(Object var1);    public native void throwException(Throwable var1);    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);    public native Object getObjectVolatile(Object var1, long var2);    public native void putObjectVolatile(Object var1, long var2, Object var4);    public native int getIntVolatile(Object var1, long var2);    public native void putIntVolatile(Object var1, long var2, int var4);    public native boolean getBooleanVolatile(Object var1, long var2);    public native void putBooleanVolatile(Object var1, long var2, boolean var4);    public native byte getByteVolatile(Object var1, long var2);    public native void putByteVolatile(Object var1, long var2, byte var4);    public native short getShortVolatile(Object var1, long var2);    public native void putShortVolatile(Object var1, long var2, short var4);    public native char getCharVolatile(Object var1, long var2);    public native void putCharVolatile(Object var1, long var2, char var4);    public native long getLongVolatile(Object var1, long var2);    public native void putLongVolatile(Object var1, long var2, long var4);    public native float getFloatVolatile(Object var1, long var2);    public native void putFloatVolatile(Object var1, long var2, float var4);    public native double getDoubleVolatile(Object var1, long var2);    public native void putDoubleVolatile(Object var1, long var2, double var4);    public native void putOrderedObject(Object var1, long var2, Object var4);    public native void putOrderedInt(Object var1, long var2, int var4);    public native void putOrderedLong(Object var1, long var2, long var4);    public native void unpark(Object var1);    public native void park(boolean var1, long var2);    public native int getLoadAverage(double[] var1, int var2);    static {
        registerNatives();
        Reflection.registerMethodsToFilter(Unsafe.class, new String[]{"getUnsafe"});
        theUnsafe = new Unsafe();
        ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class);
        ARRAY_BYTE_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);
        ARRAY_SHORT_BASE_OFFSET = theUnsafe.arrayBaseOffset(short[].class);
        ARRAY_CHAR_BASE_OFFSET = theUnsafe.arrayBaseOffset(char[].class);
        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset(int[].class);
        ARRAY_LONG_BASE_OFFSET = theUnsafe.arrayBaseOffset(long[].class);
        ARRAY_FLOAT_BASE_OFFSET = theUnsafe.arrayBaseOffset(float[].class);
        ARRAY_DOUBLE_BASE_OFFSET = theUnsafe.arrayBaseOffset(double[].class);
        ARRAY_OBJECT_BASE_OFFSET = theUnsafe.arrayBaseOffset(Object[].class);
        ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class);
        ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale(byte[].class);
        ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale(short[].class);
        ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale(char[].class);
        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale(int[].class);
        ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale(long[].class);
        ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale(float[].class);
        ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale(double[].class);
        ARRAY_OBJECT_INDEX_SCALE = theUnsafe.arrayIndexScale(Object[].class);
        ADDRESS_SIZE = theUnsafe.addressSize();
    }
}